The enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGPRT), catalyzes the transfer of the ribosephosphate moiety of PP-ribose-P to the N-9 position of hypoxanthine or guanine to form inosinic acid or guanylic acid respectively. In man, HGPRT is coded by DNA in the X- chromosome Mutations on the structural gene for this enzyme lead to the development of either the Lesch-Nyhan syndrome or a severe form of gout. The normal human enzyme has been highly purified and well characterized and a highly potent and specific antibody has been produced to it in this laboratory. The purpose of the present investigation is to use this antibody to isolate polysomes from cultured cells or brain which are synthesizing nascent HGPRT peptides. The mRNA from these polysomes will be further purified using membrane filtration or affinity chromatography and its ability to code for HGPRT will be tested in a cell free system. Possibly, HGPRT specific mRNA can then be used as a template for synthesizing the HGPRT DNA using the reverse transcriptase.